This invention relates to a brazed joint between a first part primarily of beryllium and a second part primarily of a dissimilar metal, such as copper, that is reactable with beryllium to form a brittle intermetallic compound at temperatures used in making the brazed joint. More particularly, the invention relates to a joint of this type which is low in brittle intermetallic compounds that can impair the resistance of the joint to impact loads.
Efforts have heretofore been made to braze a beryllium part to a copper, nickel, or iron part using conventional copper-containing brazing alloys, such as silver-copper and silver-copper-indium alloys. The resulting joints have been excessively brittle because the copper in the brazing alloy has reacted with the beryllium and formed copper-beryllium intermetallic compounds, which are very brittle materials.
It has been proposed to use aluminum-base brazing alloys for joining beryllium to itself and other metals, but such brazing alloys are unsatisfactory for the beryllium-to-copper joints we are concerned with because aluminum and copper react to form brittle intermetallic compounds that impair the joint. Similarly, the aluminum-base brazing alloys are not satisfactory for the beryllium-to-nickel and beryllium-to-iron joints we are concerned with because aluminum reacts with nickel and with iron to form brittle intermetallic compounds that impair the joint. And, moreover, these aluminum-base brazing alloys have such a low melting point that they cannot be used in the high temperature environment to which our joint is subjected.
Pure silver has also been proposed as a brazing material for beryllium, but the disadvantage of such brazing material is that its melting point is so high that at the required brazing temperatures the silver reacts with the beryllium to form a beryllium-silver eutectic, resulting in a loss of silver at the joint and deformation of the parts. Such reaction of the silver with the beryllium at these high temperatures can also result in the excessive formation of brittle silver-beryllium phases, impairing the impact resistance of the joint.
It has also been proposed to use a silver-lithium alloy for brazing the beryllium, but in certain applications, e.g., certain vacuum brazing applications, lithium is not a suitable component because of its high vapor pressure. The combination of the low surrounding pressure and the high brazing temperature can result in loss of an excessive amount of the lithium.
Another proposed brazing alloy for beryllium is silver-aluminum. This brazing alloy seems to have an undue resistance to flow and wetting of the beryllium surface, with the result being a weak bond to the beryllium surface.